Conodonts are extinct phosphatic microfossils that 'look like' teeth and are used extensively for dating rocks roughly 500-205 million years old. Ever since they were first described as fish teeth by C. H. Pander in 1856 they have caused arguments over how they should be classified and, nearly 150 years later, continue to do so. Read on to find out if they really are teeth, why they are so difficult to classify, give names to and even decide which way up they should be!

Images of platform, blade-like and coniform conodonts from the Museum collection. Note the presence of white matter inside and beneath the denticles of some of the specimens, a feature unique to conodonts.

For consistency, I shall refer to these individual phosphatic elements as conodonts and the creature that produced them as the conodont animal. Some consider this incorrect; you wouldn't refer to the 'cat animal' or the 'lion animal' for example. Often the individual specimens are referred to as conodont 'elements'.

What do they look like?

Conodonts are generally between 0.1mm to 2mm long, although some examples from a single deposit in South Africa measure up to 20mm. They take a variety of different forms including complex platforms, blade-like structures, simple cones and elongate bars with denticles (i.e. small teeth or tooth-like structures). Each specimen has a basal cavity and depending on preservation and species, white matter can be seen inside.

How do you find them?

Usually they are found in marine rocks (limestones or shales) and are released by dissolving them in acetic acid (the acid constituent of vinegar); a process that can take many weeks and sometimes months. The resulting residues are sieved and concentrated into a heavy fraction containing the conodonts by using a heavy liquid such as sodium polytungstate. The majority of collections consist of disarticulated remains and this is the main issue facing scientists studying their distribution.

A scanning electron microscope image of conodonts from the Silurian of Gotland, Sweden (photograph Dr Paul Taylor, NHM). Although many different shapes can be seen here, the specimens illustrated probably belong to only two species.

What is a species?

Early conodont workers described each shape encountered under a different species name as nothing was known about the animal that produced them, or even if it was an animal. Despite the later discovery of bedding plane assemblages of individual conodonts arranged in biological position, many workers continued to give separate names to each form.

In the latter stages of the 20th Century, arguments raged over whether to use multielement taxonomy, where different shaped but biologically related elements were grouped together under one species name. Some scientists preferred to continue to name each element separately and as a result, older published literature can be confusing.

A bedding plane assemblage of Idiognathodus from the Carboniferous of Bailey Falls, Illinois, USA. Fused clusters of conodonts and bedding plane assemblages like these are preserved in the fossil record only in exceptional circumstances. They give direct evidence of the biological grouping and positioning of the various elements in the conodont animal. Left: an SEM image. Right: the same specimen photographed under a light microscope. The black scale bar in the middle is about 0.5mm.

Are they teeth?

Although conodonts look like teeth, it has also been suggested that they could have functioned as sieve structures to filter fine particles. One of my favourite early interpretations of the conodont animal was published by Maurits Lindstrom in 1974. You can imagine these elongate conodonts with upper denticulated surfaces acting very much like a filter if arranged like this.

An interpretation of the conodont animal as published by Lindstrom in 1974. I like to call this the 'loo roll' reconstruction!

Polygonal patterns on the upper surfaces of some conodonts show the impressions of cells and suggest that - at least at some stage - parts of some conodonts were fully enclosed in soft tissue. Wear patterns on the surfaces of conodonts and growth studies based on bedding plane assemblages suggest that for some conodonts, the elongate denticulated conodonts were used in a rasping action to capture food and pass it backwards to more blade and platform shaped cutting and grinding teeth. However, this is not universally accepted with some scientists suggesting that conodonts could not have functioned in a cutting action.

Polygonal microsculpture representing the impressions of cells on the platform surface of the Devonian conodont Ancyrodella.

What produced them?

The conodont animal was discovered by chance in a Scottish museum in the early 1980s by some scientists looking for shrimp fossils in the Carboniferous Granton Shrimp Bed. This story is often quoted by curators trying to justify the upkeep of large collections as it is an excellent example of a major discovery resulting from an old uncatalogued collection. The discovery ended one of the longest running sagas in palaeontology; what produced the conodonts?

This is one of 10 specimens from the Granton Shrimp Bed of Edinburgh where details of the body of the conodont animal are preserved. The Museum purchased this specimen in the 1980s at around the time that the first paper on the conodont animal was published. The scale bar shows millimetres so the preserved part of the body is just over 1.5cm long.

Details from the 10 specimens available were amalgamated to produce a reconstruction of the conodont animal showing that it had an elongate body with chevron shaped muscle blocks, a caudal fin, a notochord running along its body and paired eyes. There are now other examples of soft body preservation of conodont animals including the giant conodont Promissum pulchrum from the Ordovician of South Africa. This has a very similar body plan to the Granton animals.

How should they be classified?

Although many early conodont workers were only interested in studying the stratigraphical distribution of conodonts for biostratigraphy (relative dating of rocks on the basis of their biological content), between 1876 and 1975 there were 46 different conodont affinities published. Some concluded that they were related to worms, snails, arthropods, chordates and even plants. Others considered them so different from anything else that they should represent a separate phylum, the Conodonta.

The precise interpretation of the preserved soft tissues of the conodont animal and histological sections through conodont hard tissues continues to divide the scientific community. Interpretations of conodont hard tissues as representing enamel, cellular bone and globular calcified cartilage have led many to classify them as early vertebrates placing them as more derived than the living lampreys and hagfish and precursors to the early fishes.

Not all scientists accept this because some vertebrate workers consider the tissues, particularly the conodont white matter, to be unique to conodonts and unrelated to the dentine and bone present in early fishes. This, allied to differing interpretations of the conodont soft tissues has led to suggestions that they are Chordates but unrelated to the Vertebrates.

The conodont feeding apparatus and its position within the conodont animal (boxed area) based on Idiognathodus and Clydagnathus respectively. Image courtesy and copyright of Prof. Mark Purnell, University of Leicester. See the text for an explanation of the labels.

What way up should conodonts be?

Before the discovery of the conodont animal and detailed studies of bedding plane assemblages, the exact biological positioning of conodonts within the mouth part of the conodont animal was conjectural. Various conventions used to describe anterior/posterior, upper/lower and inner/outer have subsequently proven to be incorrect. For example, in old terminology the 'anterior blade' of the P1 element is shown above to be a ventral blade.

P (Primo) elements were considered to be at the front of the mouth and S (Secundo) elements further back. Discovery of the conodont animal has shown that the reverse is true. Element terminology using the terms P, S and M is ingrained in the literature and will never be changed. However, many continue to use outdated terminology to describe anterior/posterior, upper/lower and inner/outer or use similarity of shape to infer similarity of biological positioning within the conodont animal.

Summary

I have given a very simplistic guide to conodonts here, showing some of the reasons why there have been and still are so many arguments over naming them, working out their function, classifying them and even orientating them. This post is not intended to champion the research of any particular academic or to give strong views on any of the arguments mentioned but if you are interested to receive further details of scientific literature discussing these issues then why not comment below or contact me directly.

I'm so tempted to say that a microfossil curator attends meetings and writes e-mails. Sometimes it feels like that. I decided to document a typical day back in January where e-mails and meetings helped prepare towards a loan for an art exhibition, gave news of a potentially exciting new acquisition and a possible research opportunity involving micro-CT scanning.

One of Irene Kopelman's items in the Gasworks Gallery based on microfossils from our collection

The bulk of the e-traffic involves preparations towards an exhibition that opened on 10 Feb at the Gasworks Gallery near the Oval Cricket Ground. Artist Irene Kopelman's work was partly inspired by some slides of radiolarian microfossils from our collections. We are preparing an exhibition loan of the slides and today there is a lot of correspondence discussing arrangements for two open day tours I am holding to accompany the exhibition.

Most microfossils are so small that I have to deal with images rather than the specimens themselves. We recently sent some specimens on loan to the Smithsonian Institution in Washington where a researcher has made some images for a publication and left them on an ftp site for me to collect. I am also making arrangements for other images of our specimens to be sent to us by one of our regular visitors. They have posted them on an excellent site for people interested in foraminiferal microfossils.

Aggerostramen rustica, a type of foraminiferal microfossil that builds a shell from sediment. In this case, sponge spicules have been chosen. This image has been posted on-line at the foraminifera.eu site mentioned above

Typically a day will not pass without some correspondence with future visitors to the collections and/or an actual visit from a scientist. Two visitors want to come in a couple of days time and another wants to visit the following week to discuss a short paper on a major collection of 2,500 slides that they donated last year.

In a few days time I'm off to our collections outstation in Wandsworth to meet OU PhD student Kate Salmon who is using our collections to study ocean acidification. I need to book a Museum vehicle to transport me to Wandsworth and to bring the collections back that she would like to borrow.

I mentioned meetings but you'll be glad to know that I'm not going to go into detail here. From one meeting I come away with two additional enquiries to answer; a request by a journalism student for a 5 minute mock radio interview and a student wants images of some of our specimens for their thesis.

I am also asked to assess a destructive sampling request as my boss is away. Sometimes our samples or specimens need further analysis to reveal their true scientific potential. In this case the borrower wants to make thin sections of fragments of fish fossils and to carry out 3-D imaging using a synchrotron (see my previous blog on sex in the Cretaceous for details of synchrotrons). The work will potentially give important details about early fish evolution so the request is ratified.

The excellent research facilities here at the Museum offer many exciting possibilities. Today an e-mail has come in requesting bids for use of the micro-CT scanner. I want to test whether this method can provide 3-D images of some tiny specimens the reverse sides of which we cannot analyse at the moment because they are stored embedded in wax. If it works, some 3-D images of some of our most important specimens will be delivered to the web. Some of these species have been used extensively in studies on climate change and oceanography.

One message informs me that an exciting new sample has just been sent as a donation from Oman. When it arrives I will need to dissolve some of it in acid (vinegar) to release the tiny fossils. Traces of fish microfossil are clearly visible on the surface of the rock so this sounds very promising and possibly the subject of a new paper on early fish evolution.

It would appear from everything listed above that there is not much time for any other activities. However, documenting the collections for the web is one of our core duties so I find time in the afternoon to work towards a documentation project. I am also on duty for an hour to answer questions from my fellow curators and my mentee Jacqui about using the databasing system.

A number of people including my two new colleagues Tom and Steve, pop their heads round my door to ask questions about the collections or bring me information. Retired Museum Associate Richard Hodgkinson is in today and has some questions about his project. Another retired member of staff brings me a copy of his latest paper and former volunteer and now colleague Lyndsey Douglas comes to tell me that my blog has been quoted in the January edition of the Museums Journal!

It's an amazingly variable job being a microfossil curator and no day is ever the same as another. I love my job and I think of it as unique. I don't know of anyone else in the world who has a similar job in Micropalaeontology. If you have a similar job, I'd love to hear from you.

Film crews are not an uncommon sight behind the scenes at the Natural History Museum but they have never come to see me ... until recently, that is.

Filming takes place in the Palaeontology Department regularly for documentaries, with staff interviewed or specimens brought out from their cabinets for a few minutes of fame. Usually the film crews want large vertebrates like dinosaurs or early human fossils, and one or two members of staff are well known for regular appearances in the media. For example, an episode of the BBC TV show New Tricks was filmed after hours last November and recently the department was featured in the BBC Documentary Museum of Life.

At the time I was disappointed that no aspect of micropalaeontology was featured in the BBC’s programme. So it was a pleasant surprise when I was asked to provide some specimens for a Korean film crew from EBS (Educational Broadcasting System) who were making a documentary on the early evolution of life called "The Secret Lives". They wanted to know about the earliest armoured fishes, the arandaspids.

These early fish were probably poor swimmers, scrabbling around on the bottom of the shallow sea, filtering for food. They play an important role in helping us to understand the early evolution of vertebrates. For further details including a picture of a whole arandaspid fish see:

Perhaps my most important fossil discovery was some fragments of the arandaspid Sacabambaspis in a consultancy sample sent to me from Oman in 2005. I wasn’t expecting to find fish and certainly not anything as significant as this. Luckily for me a university colleague had a large grant to study them so he funded a trip there in November 2006.

Wadi Daiqa, Oman

Because they are so small we were not expecting to see any "in the field" and were expecting to have to take back rocks to the Museum to dissolve and analyse. However, on the final day of our trip I saw by my foot a specimen with some tiny fish fragments.

The specimens themselves appear on the surface of some rocks as tiny black specks so I was amazed that I managed to see them (especially as I was told on my return that I would need glasses for reading!).

As a result of my find we went back in 2007 to collect some more samples. The image below suggests that they should be easy to spot. However, this is the very best specimen discovered after days of searching and the fragments are much larger than the original find.

The film crew first wanted a close up of the rock...

Close up of the surface of the rock being filmed. Impoverished curators use a one pence piece for scale.

Filming a close up of the arandaspid fish fragment rock in the Palaeontology Imaging Suite.

Then they wanted me to talk about its significance and say what it tells us about early fishes and their habitat.

The Oman discoveries showed that the fish were present all around the margins of the ancient continent of Gondwana and not just in the southern regions as had previously been shown by the findings from South America and Australia. In the Ordovician period about 450 million years ago, Gondwana was an amalgamation of what we currently know as Africa, South America and Australia with some parts of China and the Middle East.

Rocks from similar geological settings have produced similar fish fossils from Argentina, Bolivia and Australia so we know that this particular type of fish lived in shallow waters on the continental margins of Gondwana.

After this they wanted to film a close up of some arandaspid scales and plates on a computer screen. The filming took place in the Palaeontology Department’s imaging suite where we used a Leica microscope with a Zeiss Axiocam digital camera to provide a close up of some of the specimens that we found.

Zeiss Axiocam set up in the Palaeontology Imaging Suite.

Here I was able to show a close up of the rock on the computer and to show some features of the microscopic fragments that were inside it. I have already photographed some fragments by scanning electron microscope (SEM) so I was also able to show some of these to illustrate the close up features of some arandaspid fish scales. The “oak leaf”-like tubercles are typical of this type of fish.

Scanning electron microscope image of a scale of Sacabambaspis showing oak-leaf tubercles. The scale on the bottom is 0.38mm so the width of the scale is less than 1mm.

Sadly, I’m not sure I’ll ever see the final product as I don’t think our TV gets EBS. However, I am happy to play a small part in researching into some of the earliest vertebrates to have lived on our planet.

This is Giles Miller's Curator of Micropalaeontology blog. I make the Museum micropalaeontology collections available to visitors from all over the world, publish articles on the collections, give public talks and occasionally make collections myself.